1. Field of the Invention
The present invention relates to nonaqueous electrolyte secondary batteries, and more specifically to a nonaqueous electrolyte secondary battery which can be used as a power source for memory backup of a portable device.
2. Description of the Related Art
In recent-years, high electromotive force nonaqueous electrolyte secondary batteries using nonaqueous electrolyte have been widely used as secondary batteries of high output and high energy density. Such nonaqueous electrolyte secondary batteries are used as a power source for memory backup of portable device, as well as a main power source of portable device, and in recent years, increase in energy density is demanded not only in a main power source of portable device but also in a power source for memory backup.
As a secondary battery for memory backup, for example, a battery in which lithium cobaltate (LiCoO2) is used as a positive-electrode active material and lithium titanate (Li4Ti5O12) having spinel structure is used as a negative electrode active material has already been brought into practical use. However, the density and weight specific capacity of lithium titanate used as a negative electrode active material are 3.47 g/mL and 175 mAh/g, respectively, so that there is a problem that energy density per volume is low. In contrast, molybdenum dioxide reversely reacts with lithium in a similar potential region to that of lithium titanate and has density and weight specific capacity of 6.44 g/mL and 210 mAh/g, respectively, and has higher volume energy density than lithium titanate. Use of molybdenum dioxide as an alternative to lithium titanate enables increases in energy density per volume of battery.
For example, Japanese Patent Laid-open No. 2000-243445 proposes to use lithium-containing manganese oxide as a positive electrode active material and molybdenum dioxide for negative electrode.
A secondary battery for memory backup is mounted as a battery to be incorporated into a device, and used without a protective circuit from view points of implementation area and cost. Therefore, it is assumed that over discharge condition may occur as the condition that electric current is not supplied from the main power source lasts for a long time, and hence it is demanded that capacity decrease is small even if over discharge cycle is conducted.
As described above, molybdenum dioxide is superior in energy density per volume to lithium titanate. However, examination made by the present inventor revealed that in a nonaqueous electrolyte secondary battery in which lithium cobaltate is used as a positive electrode active material and molybdenum dioxide is used as a negative electrode active material, rapid decrease in capacity occurs with lapse of over discharge cycle, and sufficient cycle characteristic is not obtained.
Further, when molybdenum dioxide is uses as a negative electrode active material, expansion and contraction at the time of occluding and releasing of lithium are large, so that it is impossible to obtain sufficient cycle characteristic.
It is an object of the present invention to provide a nonaqueous electrolyte secondary battery which is applicable as a power source for memory backup, and having large battery capacity and excellent over discharge cycle characteristic.